[atlas] Resolves AAAA Correctly, Doesn't Resolve A - best way to resolve this?

I've been having a few problems setting up my software probe, but this has now almost stabilised. It's been quite an education in DNS setup! My problem now is that my probe General Information is reporting "Resolves AAAA Correctly, Doesn't Resolve A, IPv4 Stable 1d, IPv4 Works, Software IPv4 RFC1918, IPv4 Capable" but I can't seem to fix the "Doesn't Resolve A" problem: is there anything specific that I can try from the command line to diagnose this? FWIW, all other traceroutes, pings, digs etc on the box seem to be working.